1. Field of the Invention
The present invention relates to a reactor system for semiconductor processing and, in particular, to a dome reactor chamber.
2. Description of the Relevant Art
In the course of constructing a semiconductor wafer, a variety of procedures and processes often involve placing the wafer within an enclosed reactor chamber. For instance, the reactor chamber may be used during the growth of an epitaxial silicon layer, or the formation of a thermal oxide or thermal nitride layer over a silicon layer. Additionally, the reactor chamber is often used in high temperature and high or low pressure environments for processes such as chemical vapor depositions and thermal annealing of integrated circuit structures already formed on the wafer.
Prior semiconductor processing systems typically included a reactor chamber and a gas, heat and coolant source. The reactor chamber is used to provide a controlled environment for the desired processing of the semiconductor wafer. The gas source provides purging and reactant gases to the reactor chamber. The heating source is typically located outside of the walls of the reactor chamber and it transmits energy through the chamber walls to heat the wafer positioned within the reactor chamber. The wafer is often mounted on a support structure which serves as a susceptor to absorb energy transmitted into the chamber and to convey the resulting heat to the wafer being processed. The coolant usually flows over the outer surface of the chamber to minimize thermal expansion and distortion of the chamber.
Prior reactor chambers are frequently formed of quartz because quartz has a high melting point and low coefficient of thermal expansion. This allows the reactor chamber to withstand high temperatures, often over 1100.degree. C., used in some processes such as chemical vapor depositions (CVD) because the quartz has a melting point generally within the range of 1,300.degree. to 2,000.degree. C. The quartz also permits the heat from the heat source to transfer through the walls of the reactor chamber to heat the semiconductor wafer.
Prior reactor chambers are typically cylindrical, either over the entire chamber length or over a substantial portion of the chamber length, in order to withstand low pressure processing of the semiconductor wafer. The cylindrical reactor chamber allows for low pressure processing of the wafer because the stress caused by the pressure differential is uniformly distributed along the walls of the reactor chamber. The cylindrical reactor chamber also distributes the stress in the walls uniformly despite variations in wall thickness. The uniform distribution of stress is desirable because it reduces stress concentrations and minimizes the possibility of cracks, breaks or other damage to the walls of the reactor chamber during low pressure processing of the wafer.
The cylindrical walls, however, undesirably impair the uniform flow of the reactant gases over the surface of the semiconductor wafer because the flow rates of the gases are higher over some portions of the wafer and lower over other portions of the wafer. This causes the undesirable non-uniform coating or processing of the wafer, which impairs the integrity of the semiconductor wafer.
It is known to increase the thickness of the chamber walls to withstand the increased stress in the chamber caused by the low pressure processing of the wafer. The increasing thickness of the walls, however, undesirably increases the thermal insulation of the reactor chamber. The effectiveness of the coolant, which is typically air, in reducing the temperature of the chamber walls is reduced because it cannot effectively cool the inner surface of the chamber walls. The hotter inner surface tends to expand more rapidly than the cooler outer surface, and the uneven temperature distribution may cause the chamber walls to crack or break.
As disclosed in U.S. Pat. No. 4,920,918 issued to Adams, et al., a reactor chamber may be provided with a plurality of reinforcing gussets fixed to the outer surface of the reactor chamber to permit low pressure processing of the semiconductor wafer. The gussets provide additional strength to avoid the distortion of the walls of the reactor chamber during low pressure procedures without increasing the wall thickness of the reactor chamber. The gussets, however, undesirably impair the uniform heating of the semiconductor wafer because the gussets impede the heat from being evenly transmitted through the walls of the reactor chamber. This undesirably creates shadows or hot and cold spots within the reactor chamber which causes non-uniform heating of the wafer. The gussets also hinder the cooling of the reactor chamber because the coolant cannot pass over the entire outer surface of the reactor. Instead, the outwardly extending gussets prevent the coolant from cooling some portions of the exterior surface of the reactor chamber, and this may cause an uneven temperature distribution in the walls of the reactor chamber. Further, the quartz gussets, fourteen of which are disclosed in one embodiment of the prior reactor chamber, are typically fused to the outer surfaces of the reactor chamber. Fusing the gussets to the walls is a time consuming and costly process, and this process requires the reactor chamber be annealed at least once during the attachment of the gussets to relieve any thermally induced stresses in the reactor chamber.
It is, therefore, desirable to provide a method of making a reactor chamber that permits even heating, thermal uniformity and heat distribution within the reactor chamber, and uniform cooling of the walls of the reactor chamber.
A need therefore exists to make a reactor chamber which is useful in processing semiconductor wafers at an elevated temperature and non-ambient pressure. A dome surface provides additional strength to avoid distortion under non-ambient pressure processes, and the dome surface reduces the possibility of the reactor chamber developing cracks or breaks. Advantageously, the dome surface does not require gussets, other support members or increased wall thickness to support the reactor chamber during non-ambient pressure procedures. The dome surface also allows for uniform heating and cooling of the reactor chamber because no gussets or support members extend from the walls of the reactor chamber.
A graphite mold may be used to configure a piece of quartz into a dome shape. The graphite mold, however, creates a rough and irregular quartz surface because of the extended contact of the quartz with the graphite surfaces of the mold. The quartz surface must be polished to create a smooth surface, and the polishing of the dome surface is a time consuming and costly process. A need therefore exists to make a quartz dome having a substantially smooth surface that eliminates the need for polishing.